1. Field of the Invention
The present invention relates generally to a nuclear core component hold-down assembly and more particularly to such a hold-down assembly that is compatible with a top mounted instrumentation system that can provide a defined channel at a central location in the fuel assembly for the insertion and removal of in-core instrumentation.
2. Description of the Prior Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side.
For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
An exemplary reactor design is shown in more detail in FIG. 2. In addition to the core 14 comprised of a plurality of parallel, vertical, co-extending fuel assemblies 22, for purposes of this description, the other vessel internal structures can be divided into the lower internals 24 and the upper internals 26. In conventional designs, the lower internals function is to support and align core components and guide instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in this figure), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 2, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the vessel and the core barrel 32, is turned 180° in a lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies 22 are seated and through the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, the lower core support plate, at the same elevation as 37. The coolant flow through the core and surrounding area 38 is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially to one or more outlet nozzles 44.
The upper internals 26 can be supported from the vessel or the vessel head and include an upper support assembly 46. Loads are transmitted between the upper support assembly 46 and the upper core plate 40, primarily by a plurality of support columns 48. A support column is aligned above a selected fuel assembly 22 and perforations 42 in the upper core plate 40.
The rectilinearly moveable control rods 28 typically include a drive shaft 50 and a spider assembly 52 of neutron poison rods that are guided through the upper internals 26 and into aligned fuel assemblies 22 by control rod guide tubes 54. The guide tubes are fixedly joined to the upper support assembly 46 and connected by a split pin 56 force fit into the top of the upper core plate 40. The pin configuration provides for ease of guide tube assembly and a replacement if ever necessary and assures that the core loads, particularly under seismic or other high loading accident conditions are taken primarily by the support columns 48 and not the guide tubes 54. This support column arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability.
FIG. 3 is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character 22. The fuel assembly 22 is the type used in a pressurized water reactor and has a structural skeleton which, at its lower end includes a bottom nozzle 58. The bottom nozzle 58 supports the fuel assembly 22 on a lower core support plate 36 in the core region of the nuclear reactor. In addition to the bottom nozzle 58, the structural skeleton of the fuel assembly 22 also includes a top nozzle 62 at its upper end and a number of guide tubes or thimbles 54, which extend longitudinally between the bottom and top nozzles 58 and 62 and at opposite ends are rigidly attached thereto.
The fuel assembly 22 further includes a plurality of transverse grids 64 axially spaced along and mounted to the guide thimbles 54 (also referred to as guide tubes) and an organized array of elongated fuel rods 66 transversely spaced and supported by the grids 64. Also, the assembly 22 has an instrumentation tube 68 located in the center thereof and extending between and mounted to the bottom and top nozzles 58 and 62. With such an arrangement of parts, fuel assembly 22 forms an integral unit capable of being conveniently handled without damaging the assembly of parts.
As mentioned above, the fuel rods 66 in the array thereof in the assembly 22 are held in spaced relationship with one another by the grids 64 spaced along with fuel assembly length. Each fuel rod 66 includes nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74. The pellets 70 are maintained in a stack by a plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The fuel pellets 70, composed of fissile material, are responsible for creating the reactive power of the reactor.
It is important to manage the axial and radial power profile of the core because the power output of the reactor is limited by the hottest temperature experienced along a fuel rod 66. There is a need to keep the operating conditions below that which will result in a departure from nucleate boiling along the cladding of the fuel rod 66. Under that type of condition the heat transfer from the fuel rod 66 to the adjacent coolant deteriorates raising the temperature of the fuel rod which can result in cladding failure. A liquid moderator/coolant such as water or water containing boron, is pumped upwardly through a plurality of flow openings in the lower core support plate 36 to the fuel assembly 22. The bottom nozzle 58 of the fuel assembly passes the coolant upwardly along the fuel rods of the assembly in order to extract the heat generated therein for the production of useful work.
To control the fission process, a number of control rods 78 are reciprocally moveable in the guide thimbles 54 located at predetermined positions in the fuel assembly 22. Specifically, a rod cluster control mechanism (spider pack) 80 positioned above the top nozzle 62 supports the control rods 78. The control mechanism has an internally threaded cylindrical hub member 82 with a plurality of radially extending flukes or arms 52. Each arm 52 is interconnected to the control rods 78 such that the control rod mechanism 80 is operable to move the control rods vertically in the guide thimbles 54 to thereby control the fission process in the fuel assembly 22, under the motive power of control rod drive shafts 50 which are coupled to the control rod hubs 82, all in a well-known manner.
As previously mentioned, the fuel assemblies are subject to hydraulic forces that may exceed the weight of the fuel assemblies and therefore cause the fuel assemblies to “float” in the reactor if they are not properly secured. If a fuel assembly were to float upward just enough to cause it to be disengaged from the seating surface of the lower core plate on which it sits, it would vibrate laterally, and this condition could subject the fuel assembly to severe fretting. Because of this possibility, fuel assembly designs have included elements whose purpose is to prevent floating.
One method of preventing floating is to mount springs (86 shown in FIG. 3) on the tops of the fuel assemblies. The springs are compressed between the upper core plate and the remainder of the fuel assembly, thereby providing sufficient hold-down force to prevent the fuel assembly from being disengaged from seating surfaces on the lower core support plate. Another example of such a spring arrangement is described in U.S. Pat. No. 4,728,487. The foregoing patent describes a hold-down arrangement comprising a vertical column centrally supported on the upper surface of the top nozzle adapter plate. A spring is concentrically wound around the column and a hold-down bar (yoke) is slidably mounted on the column over the spring. The hold-down bar rests against the upper core plate when installed in the reactor and compresses the spring to hold down the fuel assembly and core component. In conventional reactor designs, such as the one described in the patent, thermal couples are positioned at the lower end of the support columns 48 and the thermal couple signal cabling are fed through the support columns and exit the reactor through penetrations in the reactor head 12, which are not shown in FIG. 2. The in-core flux detectors and other in-core instrumentation that are located in the fuel assembly instrumentation thimbles are fed through penetrations in the lower head of the reactor, the lower support plate 37 and lower core plate 36 into the instrument thimbles (also referred to as instrumentation tubes) 68 through the bottom of the fuel assemblies 22. In the conventional designs no instruments are fed into the instrument thimbles through the top of the fuel assemblies. Access to the top of the instrumentation thimbles are blocked by the hold-down arrangement described in U.S. Pat. No. 4,728,487.
The Westinghouse AP1000 reactor is a third generation-plus pressurized water reactor design. The moveable bottom-mounted in-core instrumentation has been replaced by a fixed top-mounted instrumentation system that accesses the core through penetrations in the reactor head 12. Thus, no vessel penetrations exist beneath the bottom of the core. The in-core instrumentation is important for providing an in-core flux map and signals necessary for monitoring core exit temperatures of the reactor core, which are used to calibrate neutron detectors and to optimize core performance.
Accordingly, a new design is required to access the instrument thimbles 68 from the top of the fuel assembly 22 and provide a centering alignment and shielding the instrumentation components from cross flow. Such a design is desired that will provide effective shielding with minimal changes to the conventional hold-down devices.